Gas turbine engines operate by passing a volume of high energy gases through a plurality of stages of vanes and blades, each having an airfoil, in order to drive turbines to produce rotational shaft power. The shaft power is used to turn a turbine for driving a compressor to provide air to a combustion process to generate the high energy gases. Additionally, the shaft power is used to power a secondary turbine to, for example, drive a generator for producing electricity, or to produce high momentum gases for producing thrust. In order to produce gases having sufficient energy to drive both the compressor and the secondary turbine, it is necessary to combust the air at elevated temperatures and to compress the air to elevated pressures, which again increases the temperature. Thus, the vanes and blades are subjected to extremely high temperatures, often times exceeding the melting point of the alloys comprising the airfoils.
In order to maintain the airfoils at temperatures below their melting point it is necessary to, among other things, cool the airfoils with a supply of relatively cooler bypass air, typically siphoned from the compressor. The bypass cooling air is directed into the blade or vane to provide impingement and film cooling of the airfoil. Specifically, the bypass air is passed into the interior of the airfoil to remove heat from the alloy, and subsequently discharged through cooling holes to pass over the outer surface of the airfoil to prevent the hot gases from contacting the vane or blade. Various cooling air patterns and systems have been developed to ensure sufficient cooling of the trailing edges of blades and turbines.
Typically, each airfoil includes a plurality of interior cooling channels that extend through the airfoil and receive the cooling air. The cooling channels typically extend straight through the airfoil from the inner diameter end to the outer diameter end such that the air passes out of the airfoil. In other embodiments, a single serpentine cooling channel winds axially through the airfoil. Cooling holes are placed along the leading edge, trailing edge, pressure side and suction side of the airfoil to direct the interior cooling air out to the exterior surface of the airfoil for film cooling. In order to improve cooling effectiveness, the cooling channels are typically provided with trip strips and pedestals to improve heat transfer from the airfoil to the cooling air. Trip strips, which typically comprise small surface undulations on the airfoil walls, are used to promote local turbulence and increase cooling. Pedestals, which typically comprise cylindrical bodes extending between the airfoil walls, are used to provide partial blocking of the passageway to control flow. Various shapes, configurations and combinations of trip strips and pedestals have been used in an effort to increase turbulence and heat transfer from the airfoil to the cooling air. However, pedestals used at the same location as trip strips, such as in U.S. Pat. No. 6,290,462 to Ishiguro et al., produce dead zones in the cooling air flow that interferes with the effectiveness of the trip strips. Pedestals are therefore typically positioned several lengths upstream or downstream of trip strips, such as disclosed in U.S. Pat. No. 5,288,207 to Linask. There is a continuing need to improve cooling of turbine airfoils to increase the temperature to which the airfoils can be exposed to increase the efficiency of the gas turbine engine.